Rocket motor casings are conventionally provided with an inner or outer heat insulating surface such as rubber. It has been proposed to adapt rubber extruder and wrapping equipment developed for the large tire fabrication industry to the construction of rocket motor insulators. The general concept of forming such components by wrapping is not new. The advantage of this approach is that the extruder would provide freshly mixed, hot, uncured rubber. When hot the rubber is tacky and is more readily processed than the same material in calendered form at room temperature. The invention disclosed herein is unique because it employs a modified roll mill which configures the extruded rubber strip to the final shape required for wrapping.
The prior art wrapping system referred to above employs a two roll mill in conjunction with the extruder. The mill puts the finishing touches on the shape of the extruded strip of uncured rubber prior to wrapping. The final strip shape which is produced is fixed and cannot be changed without shutting down and changing both the roll mill and the extruder die. This existing equipment is only capable of wrapping cylindrical or conical shapes of constant wall thickness. Tapered shapes must be wrapped with multiple layers, using excess material, and then finish-machined to the required wall thickness.
The utility and usefulness of this basic concept would be expanded exponentially if the shape of the strip could be varied during continuous equipment operations. The ability to vary strip shape would permit elimination of the very expensive final machining operations and permit the manufacture by wrapping of net, close tolerance, tapered wall thickness, rubber components that are surfaces of revolution--cylinders, domes, cones, etc. Conventionally, the strip is wrapped onto a surface of revolution comprising the surface of a rotating mandrel and it is most important that the strip nest properly with both the mandrel surface and with adjacent portions of the strip. This is a particularly crucial problem when wrapping over a curved surface, such as a dome, and where the insulation thickness must taper. If the strip is not the proper shape, it will not nest properly and the resulting layup will contain excessive voids, in the nature of space between the strips, resulting in unpredictable deformation during the curing step and loss of thickness control.